The HIV-1 Gag proteins are synthesized as a polyprotein precursor, Pr55Gag, that is cleaved by PR during virus release from the infected cell. PR-mediated Pr55Gag processing follows a sequence of events that is kinetically controlled by the rate of processing at individual cleavage sites. Completion of the Gag processing cascade is essential for virus maturation and infectivity. In collaboration with Panacos Pharmaceuticals, we found that 3-0-(3'-3'-dimethylsuccinyl) betulinic acid (PA-457 or bevirimat) potently inhibits HIV-1 maturation by specifically blocking a late step in the Gag processing pathway. We played a central role in defining the mechanism of action of bevirimat and have isolated and characterized a number of resistant HIV 1 variants. We are also investigating the target and mechanism of action of a compound (PF-46396) developed by Pfizer that is structurally distinct from bevirimat but like bevirimat blocks CA-SP1 processing. In ongoing studies, we are evaluating the antiviral activity of compounds that disrupt HIV-1 maturation by binding to the capsid protein. This work is being done in collaboration with Drs. A. Debnath and M. Summers. [Corresponds to Freed Project 4 in the April 2007 site visit report of the HIV Drug Resistance Program]